Schistosomiasis and filariasis remain formidable world public health problems, affecting the socioeconomic development of many countries in which the U.S. has a direct interest. It is recognized that effective chemotherapy is a vitally important component in the total effort to control these parasitic diseases. The exploitation of known biochemical and physiological differences between these parasites and their mammalian hosts offeres a logical approach to the development of drugs with greater selective toxicity against the parasites. Our biochemically oriented investigations of schistosomes and filariae have revealed that certain peculiarities of their nucleic acid and folate metabolism are potentially exploitable by appropriate antimetabolites, and there is reason to hope that other such metabolic peculiarities will be discovered. A particularly attractive area for further exploration which has emerged from our research is the use of host red cells as delivery vehicles for antischistosomal drugs, taking advantage of the high rate of red cell ingestion by these trematodes. A most desirable advnace in this area would be to discover or to design antischistosomal drugs which could be administered orally and which would, when entering the bloodstream, be selectively concentrated within red cells. Although the absolute dependency of filariae upon the salvage pathway for purines, their very active folate metabolism, and certain features of their pyrimidine metabolism theoretically are exploitable by elective chemotherapy, a better understanding of the pertinent metabolic pathways is required before it can be decided whether such exploitation is feasible.